Third generation partnership project (3GPP) mobile communication systems based on a wideband code division multiple access (WCDMA) radio access technology are widely spread all over the world. High-speed downlink packet access (HSDPA) that can be defined as a first evolutionary stage of WCDMA provides 3GPP with radio access technique that is highly competitive in the mid-term future. However, since requirements and expectations of users and service providers are continuously increased and developments of competing radio access techniques are continuously in progress, new technical evolutions in 3GPP are required to secure competitiveness in the future. Reduction of cost per bit, increase of service availability, flexible use of frequency bands, simple structure and open interface, proper power consumption of a user equipment (UE), and the like are defined as requirements.
A wireless communication system is different from a wired communication system in that seamless services have to be provided to a UE having mobility. That is, the wireless communication system has to support the UE which moves from one cell to another cell. When the UE is moving away from a previous base station (BS), to which the UE is currently connected, while approaching a new BS, there is a need to perform a process of changing an access point of the UE to the new BS over a network. The previous BS is referred to as a source BS. The new BS is referred to as a target BS. The process of changing the access point from the source BS to the target BS is referred to as handover. In general, in the handover, data cannot be transmitted to and received from the UE after the UE is disconnected from the source BS until the UE is connected to the target BS.
All user data has a time limit. For example, in the case of voice call, one piece of voice information has to be transmitted to a recipient within a predetermined time period. In addition, packet data (e.g., transmission control protocol/Internet protocol (TCP/IP) packet) has to be delivered from a caller to the recipient within the predetermined time period, and the recipient has to transmit an acknowledgement to the caller.
A UE and a BS continuously exchange acknowledgment information for transmitted and received data. If a single TCP/IP packet is lost in transmission by subordinate entities, a data transfer rate decreases sharply. For example, when the single TCP/IP packet is lost while data is transmitted and received at a data transfer rate of 100 Mbit/s, the data transfer rate may abruptly decrease, for example, to 10 Kbit/s. Therefore, in order to reduce influence of packet loss which may occur in the transmission of the TCP/IP packet, the wireless communication system uses a lossless mode. The lossless mode can be provided by acknowledged mode (AM) radio link control (RLC) layer. When an acknowledgement response is not received within a predetermined time period after data is transmitted or when non-acknowledgement information is received, an AM RLC entity of a transmitter retransmits the data. The transmitter does not always retransmit the data. Rather, the transmitter retransmits the data upon receiving the acknowledgment response within a predetermined maximum transmission delay time.
FIG. 1 is a flow diagram showing a handover procedure.
Referring to FIG. 1, a UE transmits data blocks to a source BS to which the UE is currently connected (step S10). The UE transmits five data blocks DB1 to DB5 in a data block unit. The source BS fails in receiving the third data block DB3. Thus, four data blocks DB1, DB2, DB4, and DB5 are stored in a buffer. When a predetermined requirement is satisfied, the source BS transmits to the UE a handover command message that instructs handover (step S11).
Thereafter, among the data blocks received by the source BS from the UE, the source BS delivers to an upper network the consecutively received data blocks DB1 and DB2 (step S12). Further, the source BS delivers the remaining data blocks DB4 and DB5 to a target BS (step S13).
The UE requests the target BS to perform synchronization (step S14). The target BS delivers timing information and uplink allocation information to the UE (step S15). The UE sends a handover confirm message to the target BS (step S16). The target BS sends acknowledgement information to the UE (step S17). The acknowledgement information indicates whether a specific data block is successfully or unsuccessfully received by the target BS from the source BS. The UE transmits to the target BS the data block DB3 that is not successfully received by the source BS (step S18). The target BS sends to the upper network the received data block DB3 together with the data blocks DB4 and DB5 previously received from the source BS (step S19).
When data is successfully received by the source BS but is not consecutively received, the source BS delivers the data to the target BS. The data is eventually transmitted to the upper network. However, the data transmission from the source BS to the target BS results in increase in processing capacity of the target BS and also increase in wired network traffic.